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Chlorine Lock Out

We are experiencing what the pool store has told us is "chlorine lock out" I have added 18 lbs of shock to the pool over the past 3 weeks and still am not getting a chlorine reading when testing the water. The pool is 33000 gallons. Can anyone tell me how to get past this. We have had the pool 15 years and NEVER had any problems.

30,000 gal concrete
10ft deepend 4ft shallow
built in 56, filter and pump in the house!!! all brass lines

Re: Chlorine Lock Out

"Chlorine lock out" is somewhere between misleading and a myth. In practice it seems to be what people say when they can't explain what is happening.

It would be a huge help if you could describe what the water looks like and post a full set of test results.

Needing to use a huge amount of chlorine at the start of the pool season to get any FC level at all is a known thing, though it isn't all that common. It usually occurs when CYA has been broken down by bacteria over the winter into ammonia. It can take a fair bit of chlorine to clean up the ammonia in the spring.

Re: Chlorine Lock Out

The first step is to raise the PH to at least 7.2. Keep in mind that the PH test will show anything below 6.8 as 6.8, so you may need to add more chemical than it would at first appear.

After doing that, I suggest trying an Extreme Chlorine Demand Test. That will give you a fairly good idea what you are up against. If you have, or can easily get an ammonia test, a good alternative to that test would be to get a measure of your ammonia level. For most people doing the extreme demand test is easier than locating an ammonia tester.